SiC single crystals are thermally and chemically extremely stable, are superior in mechanical strength, are resistant to radiation, in addition, have superior properties such as high breakdown voltage and high thermal conductivity in comparison to Si (silicon) single crystals, enable easy p- and n-conductivity electronic control by adding impurities, and have wide forbidden bandwidths (approximately 3.3 eV for 4H-type single crystal SiC, approximately 3.0 eV for 6H type single crystal SiC). Therefore, realization of high temperature, high frequency, voltage resistance/environment resistance which could not be realized with Si single crystals, GaAs (gallium arsenide) single crystals, and other existing semiconductor materials is possible. Expectations of these as next generation semiconductor materials are rising.
Conventionally, the gas phase technique and the liquid phase technique are known as typical growth methods of SiC single crystals. As the gas phase technique, normally the sublimation technique is used. The sublimation technique comprises arranging SiC material powder and an SiC single crystal seed crystal facing each other in a graphite crucible and heating the crucible in an inert gas atmosphere to epitaxially grow the single crystal. However, it is known that with this gas phase technique, the polycrystals growing from the crucible inner walls negatively affect the quality of the SiC single crystal.
Further, the liquid phase technique comprises using an SiC single crystal production system which has a basic structure comprised of a crucible for holding a solvent, for example, a graphite crucible, a solvent, a high frequency coil or other external heating system, an insulator material, a seed crystal support member that can be lowered and raised (for example, a graphite holder), and a seed crystal attached at the tip of the seed crystal support member, dissolving a C (carbon) from a C supply source, for example, the graphite crucible, into an Si melt, an Si alloy melt in which metal had been dissolved, or another Si-containing melt in the crucible to obtain a solvent, and growing an SiC single crystal layer on the SiC seed crystal through solution precipitation.
In such a method of growing SiC single crystals by the liquid phase technique, use is made of either the SiC single crystal growth method of the method of growth by providing a temperature gradient to the solvent so that the solution temperature around the seed crystal becomes lower than the solution temperature at other parts or the method of growth by slowly cooling the entire solvent, however, it is known that neither are able to avoid formation of crystals other than the single crystal due to the temperature distribution and concentration distribution in the solution at the time of cooling the solvent.
Therefore, a single crystal production system that is able to prevent or suppress formation of crystals other than the single crystal is in demand.
For example, International Patent Publication 2000-39372 describes a growth method and growth system of an SiC single crystal using a gas phase technique provided with a holding material for holding a seed crystal in an SiC seed crystal holding means comprised of a member which protrudes from the container wall surface to the growth container interior and has a higher heat radiating property than the container wall surface of the growth container. Further, the publication also describes, as a holding material with a high heat radiating property, pyrographite carbon (carbon material, thermal conductivity of high thermal conductivity direction: 120 kcal/mhr° C.) which can be used in a graphite crucible lid with anisotropic thermal conductivity.
Further, Japanese Patent Publication (A) No. 2006-62936 describes as prior art an SiC single crystal production system using a gas phase technique able to prevent seed crystal breakage and detachment by interposing a plastic deformable carbon sheet between a seed crystal carrier and seed crystal to absorb the thermal stress generated at the interface.
Further, with respect to the heat conducting anisotropic material, Japanese Patent Publication (A) No. 10-24362 describes a tip of a soldering iron comprising a flexible sheet of a carbon-based material wrapped around the outer peripheral surface of a core to form a laminated body. Further, the publication describes as a specific example of a flexible sheet of a carbon-based material, a graphite sheet having anisotropic thermal conductivity.
Further, Japanese Patent Publication (A) No. 2007-305700 describes a heating member suitable for use in heating an electronic device comprising a sheet member a having a thermal conductivity in the thickness direction higher than a thermal conductivity in the in-plane direction and a sheet member b having a thermal conductivity in the in-plane direction higher than a thermal conductivity in the thickness direction laminated so that the sheet member a is positioned at the side close to the heat radiating part.
Further, Japanese Patent Publication (A) No. 2009-55021 describes a thermally conductive sheet comprising a plurality of thermally conductive sheets having a graphite phase containing anisotropic graphite particles that are laminated to each other by bonding, then cut in the direction perpendicular to the lamination direction and able to be used as a heat radiating member of an electronic device.
Further, Japanese Patent Publication (A) No. 2009-4385 describes a semiconductor package in which an anisotropic thermally conductive graphite sheet is arranged at the heat radiating region of a semiconductor element.
However, even if applying the arts described in these known documents in a solution growth of single crystal production system, it is not possible to achieve crystal growth with no problems in preventing or suppressing formation of crystals other than the single crystals so as to grow the single crystal with a high growth rate.
That is, there are several problems interfering with single crystal growth in the area around the seed crystal when bringing the seed crystal in contact with a high temperature solution, for example, a solvent heated to a high temperature exceeding 1600° C., for example, around 2000° C., and during growth to make the seed crystals grow with a high growth rate. For example, at the bonding surface of the seed crystal and graphite tip at the time of contact with the most initial solution for beginning crystal growth, cracking of the seed crystal and peeling from the graphite axis occur impeding the growth process. Even in the event that the seed crystal does not detach, the majority of the bonding surface of the seed crystal will peel off making it difficult to obtain normally grown crystal.
Further, during growth, the solvent is agitated by stirring etc., so the portion of the graphite axis which the seed crystal is bonded to becomes wetted by the solution at time of contact with the solution and during growth. The carbon from that portion reacts with the solution causing SiC nucleation. The SiC crystals nucleated at the axis surface, that is, polycrystals, gradually increase to the point of hindering the original object, that is, single crystal growth from the seed crystal, thus posing a large problem to single crystal growth of the liquid phase technique.
Further, Japanese Patent Publication (A) No. 2004-269297 describes a gas phase method-based SiC single crystal production system where a flexible carbon sheet is interposed between a seed crystal and a base for holding the seed crystal to ease the stress due to the difference in thermal expansion between the seed crystal and the base, and Japanese Patent Publication (A) No 2004-338971 describes, for an SiC single crystal growth method and growth system thereof, a gas phase method-based SiC seed crystal production system having a buffer member comprised of a carbon sheet arranged between an SiC seed crystal and a seed crystal support member for supporting the seed crystal and thereby able to mitigate the difference in the thermal expansion between the seed crystal and the seed crystal support member. Further, the publication describes, as a specific example of a carbon sheet, one of about 25% of the thermal conductivity of the SiC seed crystal.
Application of such a carbon sheet to a solution method-based SiC single crystal production system may also be considered. However, with a carbon sheet, normally the sheet surface is arranged parallel to the bonding surface, so the thermal conductivity in the direction perpendicular to the solution surface of the solvent (that is, the graphite axis direction) is low and negatively affects the growth rate. Further, if using a thermally conductive anisotropic material (solid carbon), breakage, peeling, or detachment of the seed crystal is liable to occur due to the difference in the thermal expansions between the seed crystal and the thermally conductive anisotropic material.